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The Birth of Stars and Protoplanetary Systems

For centuries, astronomers and philosophers wondered how our solar system and its planets came to be. As telescopes advanced and space probes were sent out to explore, we learned more and more about our solar system, which gave us clues to how it might have taken shape. 

But were our ideas right?

We could only see the end result of planet formation, not the process itself. And we had no other examples to study. Even with the knowledge gained about our solar system, we were left to wonder, are there other planetary systems out there, and did they form like ours? Discoveries made by the Hubble Space Telescope are helping us fill in key pieces to the puzzle of how planets form.

According to our current understanding, a star and its planets form out of a collapsing cloud of dust and gas within a larger cloud called a nebula. As gravity pulls material in the collapsing cloud closer together, the center of the cloud gets more and more compressed and, in turn, gets hotter. This dense, hot core becomes the kernel of a new star.
Meanwhile, inherent motions within the collapsing cloud cause it to churn. As the cloud gets exceedingly compressed, much of the cloud begins rotating in the same direction. The rotating cloud eventually flattens into a disk that gets thinner as it spins, kind of like a spinning clump of dough flattening into the shape of a pizza. These "circumstellar" or "protoplanetary" disks, as astronomers call them, are the birthplaces of planets.

As a disk spins, the material within it travels around the star in the same direction. Eventually, the material in the disk will begin to stick together, somewhat like household dust sticking together to form dust bunnies. As these small clumps orbit within the disk, they sweep up surrounding material, growing bigger and bigger. The modest gravity of boulder-sized and larger chunks starts to pull in dust and other clumps. The bigger these conglomerates become, the more material they attract, and the bigger they get. Soon, the beginnings of planets — "planetesimals," as they are called — are taking shape.

In the inner part of the disk, most of the material at this point is rocky, as much of the original gas has likely been gobbled up and cleared out by the developing star. This leads to the formation of smaller, rocky planetesimals close to the star. In the outer part of the disk, though, more gas remains, as well as ices that haven't yet been vaporized by the growing star. This additional material allows planetesimals farther from the star to gather more material and evolve into giants of ice and gas.
As each planetesimal grows bigger, it starts clearing out the material in its path, snatching up nearby, slow-moving rubble and gas while gravitationally tossing other material out of its way. Eventually, the debris in its path thins out and the planetesimal has a relatively clear lane of traffic around its star.

Protoplanetary Origins Hundreds of these planetesimals are forming at the same time, and inevitably they meet up. If their paths cross at just the right time and they're moving fast enough relative to each other, SMASH! — they collide, sending debris everywhere. But if they slowly meander toward one other, gravity can gently draw them together. They form a union, merging into a larger object. If the participants are farther apart, they might not physically interact but their gravitational encounter can pull each body off course. These wayward objects start to cross other lanes of traffic, setting the stage for additional collisions and other meetings of the rocky kind.
After millions of years, countless encounters between these planetesimals have cleared out much of the disk's debris and have built up much larger — and many fewer — objects that now dominate their regions. A planetary system is reaching maturity.


  1. So from what I have read and understand, stars form out of nebulas that shrink down to this one point which will become the star. At about 5 million kelvin, fusion begins and the fusion pressure outwards blows the remains of the nebula away.

    Now, how come some stars become so massive if the fusion process blows away the rest of the nebula? Wouldn't every star become about just as massive?

    1. Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by a plethora of observations, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 solar masses, however, the mechanism of star formation is not well understood.

      Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this radiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses. Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar. Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.

      There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks. Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competitive accretion, which suggests that massive protostars are "seeded" by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region.

      Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.


  2. I don't think that the early formation of universe was quite bound by the laws of physics as we understand them today. It probably took time for the universe to begin to cool down before elementary particles like protons and neutrons could have formed. Perhaps it was during this early stage that a period of randomness led to diverse patterns emerging that would later eventually go on to shape the universe as we know it is today, thus leading to the birth of our sun in the milky way, local group, virgo cluster - virgo supercluster ect....

  3. It really depends whether you are talking about the "entire" universe or the "observable" universe. It also depends on what you are interested in mapping: galaxies, stars, planets, etc.

    The observable universe is generally measured by the time that light has traveled since the big bang. This has been accurately measured at 13.7 billion light years in every direction. However, this distance does not represent the radius of the observable universe, for a variety of reasons, one of which is expansion. The best way to visualize the observable universe is by looking at the WMAP data which images microwave background radiation (the echo of the big bang) in every direction.

    The actual size of the entire Universe is currently indeterminable due to "rapid expansion theory." This theory states that the universe expanded to an indescribably large size (perhaps millions of times larger than the observable universe) in the first seconds after the big bang. This theory arose due to the observation that the universe does not appear to curve in on itself. Therefore, there is no telling how far it extends, or whether the universe has an "edge."

  4. it is uneven now because it was also uneven in the early universe. nothing is perfect including the distribution of, i wont say matter because there was none, particles just after the BB. these inhomogeneities amplified over time due to gravity to form the first stars and then the first galaxies.


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